Conventionally, there have been discussed and developed a secondary battery such as a lithium-ion battery, and an electrochemical capacitor such as an electric double layer capacitor and a hybrid capacitor as a storage device to be mounted in a hybrid vehicle or a fuel cell vehicle.
Such a storage device generally has a positive electrode, a negative electrode, a separator interposed between the electrodes, and a cell bath accommodating the electrodes and the separator, and filled with an electrolyte solution in which these components are immersed. In each electrode, charge/discharge of the storing device is carried out by discharge of energy stored by the electric double layer and/or an oxidation-reduction reaction.
However, in the case where an organic electrolyte solution prepared by dissolving a lithium salt such as, for example, lithium hexafluorophosphate (LiPF6) in an organic solvent such as ethylene carbonate (C3H4O3) is used, when a high voltage (for example, not less than 4.23 V vs. Li/Li+) is applied on the positive electrode, hydrofluoric acid (HF) is generated in a charging/discharging cycle, and the HF causes production of lithium fluoride (LiF) on the negative electrode. The LiF covers the negative electrode, which may in turn deteriorate capacitance of the negative electrode and characteristics of the charging/discharging cycle., thereby lowering the energy density stored in the storage device.
In order to eliminate the above-mentioned disadvantage, for example, a hybrid capacitor comprising a separator made of ceramic filters, a positive electrode made of a mixture of KOH-activated soft carbon, KETJENBLACK ECP and PTEF dispersion blended at a weight ratio of 85:5:10, a negative electrode made of a mixture of a artificial graphite, a soft carbon and PVdF blended at a weight ratio of 22.5:67.5:10, an electrolyte solution prepared from ethylene carbonate+diethyl carbonate solvent containing LiPF6, and a scavenger made of a mixture of Li2CO3 powder and PTFE blended at a weight ratio of 80:20 has been proposed (see, for example, Patent Document 1 (Example 2)).
Regarding the hybrid capacitor described in Patent Document 1, even when a high voltage (for example, not less than 4.23 V vs. Li/Li+) is applied on the positive electrode, deterioration of the negative electrode can be suppressed, thereby giving density growth of energy.